Realizing Locally Excited Intermediate Triplets via Naphthyl Unit in Donor‐Acceptor Based Thermally Activated Delayed Fluorescence Emitters for Efficient OLEDs

An in‐depth understanding of locally excited (LE) intermediate states is crucial in regulating the reverse intersystem crossing (RISC) process and overall efficiency of thermally activated delayed fluorescence (TADF) emitters, directing rational material synthesis. Herein, we investigated the effect of incorporating a low‐triplet‐energy naphthyl core at the acceptor, donor, and both positions through the design and synthesis of three donor‐acceptor (D‐A) emitters, with tunable optoelectronic properties. Doping 7 wt.% of NP‐ m DPA, 3BPy‐ m PNP, and NP‐ m PNP in 3,3′‐di(9 H ‐carbazol‐9‐yl)‐1,1′‐biphenyl ( m CBP) host resulted in the first excited singlet‐triplet energy gap (Δ E ST ) of 0.28, 0.31, and 0.42 eV. Temperature‐dependent transient photoluminescence (PL) measurements indicate robust TADF behavior in NP‐ m DPA and 3BPy‐ m PNP, while NP‐ m PNP exhibits weaker TADF accompanied by prominent phosphorescent fingerprints. In multi‐layer organic light‐emitting diodes (OLEDs) with an emissive layer (EML) comprising 7 wt.% of emitters in m CBP host, NP‐ m DPA exhibited an electroluminescence (EL) peak at 502 nm with the highest maximum external quantum efficiency (EQE max ) of 18.6%. In contrast, 3BPy‐ m PNP (505 nm) and NP‐ m PNP (492 nm) showed reduced EQE max of 12.6% and 5.1%. Detailed theoretical calculations reveal that incorporating a naphthyl unit in both D and A produces intermediate LE triplet states and improves RISC. Whereas, the naphthyl unit in only D or only A results in reduced Δ E ST and , enabling NP‐ m DPA to be the most efficient among the studied emitters.